Method for limiting power of a multi-stage compressor and a compressor for carrying out the method

ABSTRACT

Known compressors are matched to a predetermined power range and therefore cannot supply consumers of a closed air supply system as well as an external consumer with an adequate quantity of air. The invention is directed to a method for limiting the power of a multi-stage compressor wherein at least one compressor stage is switched off when a predetermined power limit is reached. In this way, the lowest compressor stage can be so selected that the external consumer obtains an adequate volume flow. The compressor for the method includes a bypass line ( 22 ) which bypasses the inlet valve ( 19 ) of a low-pressure chamber ( 11 ) and thereby connects the low-pressure chamber ( 11 ) to its upstream intake space ( 16 ). A controllable control valve ( 23 ) is disposed in the bypass line ( 22 ) and realizes the connection of the low-pressure chamber ( 11 ) to the intake space ( 16 ) at a predetermined opening pressure.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of German patent application no. 103 21 771.1, filed May 15, 2003, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention relates to a method for limiting the power of a compressor which compresses an airflow in at least two compression stages.

BACKGROUND OF THE INVENTION

[0003] As a rule, a corresponding air supply system is designed as a closed loop and comprises primarily the following: several air spring damping units, for example, for supporting the bodywork; one or several pressurized air stores; several control units assembled to a valve block; and, a multi-stage compressor having an air drying unit. The multi-stage compressor has the task of moving compressed air from individual air spring damping units into other air spring damping units and moving air out of the air spring damping units into the pressurized-air store or in the opposite direction from the pressurized-air store to the air spring damping units or even filling the pressurized-air stores with fresh air from the atmosphere. This multi-stage compressor is often also utilized to supply one or several consumers with pressurized air. The consumers lie outside of the closed loop. For example, tires can be filled with air or a driver's seat can be adjusted. For this purpose, fresh air from the atmosphere is utilized and not the already compressed and dried air from the pressurized-air store.

[0004] One such multi-stage compressor with two compressor stages is disclosed in German patent publication 197 15 291. This compressor comprises a compressor housing wherein a cylindrical low-pressure chamber having a larger diameter and a high-pressure chamber having a smaller diameter are configured. A piston unit having a larger low-pressure piston and a smaller high-pressure piston is mounted in the compressor housing. The low-pressure piston and the high-pressure piston are configured as one piece via a piston rod. The low-pressure chamber has an inlet valve and the high-pressure chamber has an outlet valve and both pressure chambers are connected to each other via an overflow channel. A shut-off valve is mounted in the overflow channel and opens in the direction toward the high-pressure chamber. The compressor housing is configured as a crankcase in the region of the piston rod. In this region, an electrically-operated crank gearing is disposed which engages the piston unit. The crankcase is, however, also equipped with an intake space which is connected at the input end to a feed connection for connecting to the pressurized-air store of the air supply system or to the atmosphere and, on the other hand, has a connection to the low-pressure chamber via the inlet valve.

[0005] The two-stage compressor is so configured that the power take-up of both compressor stages is matched to the power output of the electric drive unit so that the two-stage compressor has only a limited piston displacement. This limited piston displacement, however, leads to the situation that the compressor, at a low prepressure, produces only a small compressor flow which, as a rule, is not sufficient for supplying an external consumer. This is always the case when the compressor inducts its air from the atmosphere. This greatly limits the area of application of the compressor.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to limit the power of a multi-stage compressor in its upper power range to a predetermined power limit. It is also an object of the invention to provide a multi-stage compressor which is suitable for supplying different consumers.

[0007] The method of the invention is for limiting power of a compressor which compresses an air flow in at least two compression stages. The method includes the step of causing the pressure in one of the compression stages to be adapted or equalized to the pressure in the other one of the compression stages when a predetermined power limit of the compressor is reached.

[0008] The method and compressor of the invention eliminate the above-mentioned disadvantages of the state of the art.

[0009] The method of the invention especially affords the advantage that now a compressor can be utilized with any desired power capacity without exceeding the power limit of the electric drive. This same advantage is afforded by the compressor of the invention. This saves a complex matching of the power ranges of the electric drive and of the compressor.

[0010] Now there is the possibility to intentionally select a compressor which, for example, is over dimensioned relative to the power limit of an electric drive so that, in the lower power range of the compressor, a volume flow adequate for the supply of an external consumer can be generated and without the danger that the power is exceeded. This expands the area of use of the compressor because now, with the same compressor, all consumers of a closed air supply system as well as consumers, which lie outside of the closed air loop, can be supplied.

[0011] These advantages are realized with a relatively low level of technical complexity. Only a bypass line is needed between the compressor stage, which is to be switched off, and the upstream low-pressure chamber in which a simple controllable pressure valve is mounted.

[0012] It is also an advantage that the pressure valve is controllable in dependence upon different parameters because, in this way, the area of application is expanded. It is also an advantage when the pressure valve is designed as a control valve because then, the pressure valve opens continuously and the transition from the two-stage or multi-stage to a single-stage operation takes place continuously.

[0013] It is also an advantage that the multi-stage compressor can have two and more compressor stages which, in turn, makes possible a wide area of application.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will now be described with reference to the drawings wherein:

[0015]FIG. 1 shows a schematic of a two-stage compressor;

[0016]FIG. 2 is a schematic of a control valve mounted in the compressor;

[0017]FIG. 3 is a schematic showing the compressor equipped with an electrically controlled control valve; and,

[0018]FIG. 4 is a schematic showing the compressor equipped with a pneumatically controlled control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0019] The two-stage compressor comprises a compressor housing 1 which is closed pressure-tight at its ends with a larger housing cover 2 and a smaller housing cover 3. A through interior space 4 is formed within the compressor housing 1 and this space is stepped with respect to its diameter and, in this way, defines a cylindrical hollow space 5 and a cylindrical hollow space 6. The hollow space 5 is greater in diameter and the hollow space 6 is smaller in diameter. Both cylindrical hollow spaces 5 and 6 are arranged along a common axis. A stepped piston 7 with a larger low-pressure piston 8, a smaller high-pressure piston 9 and a common piston rod 10 is disposed in the through inner space 4 of the compressor housing 1. The larger cylindrical hollow space 5 and the low-pressure piston 8 form a low-pressure chamber 11 having a greater piston displacement and the smaller cylindrical hollow space 6 and the high-pressure piston 9 form a high-pressure chamber 12 having a smaller piston displacement. The low-pressure chamber 11 is configured as a first compressor stage and the high-pressure chamber 12 is configured as a second compressor stage. Both pressure chambers 11 and 12 are functionally connected via an axial overflow channel 13 which is disposed in the stepped piston 7 and extends completely through the piston rod 10.

[0020] The stepped piston 7 in combination with the inner space 4 of the compressor housing 1 forms a free space between the low-pressure piston 8 and the high-pressure piston 9. This free space accommodates a crank gear 14. The stepped piston 7 is connected to an electric drive unit 15 via this crank gear 14 so that the stepped piston 7 is configured to be axially displaceable in the low-pressure chamber 11 and, in the same manner, in the high-pressure chamber 12. In this way, the low-pressure chamber 11 and the high-pressure chamber 12 are changeable with respect to their volumes.

[0021] The free space, which is disposed between the low-pressure piston 9 and the high-pressure piston 10, is configured as an intake space 16 closeable pressure-tight to the outside. The intake space 16 can be connected at the input end via a directional valve (not shown) either to a pressurized-air store or to the atmosphere. The intake space 16, the low-pressure chamber 11 and the high-pressure chamber 12 are connected to each other in a special way. For this purpose, the compressor has an inlet connection 17 which leads to the intake space 16. The intake space 16 is connected via an inlet valve 19 to the low-pressure chamber 11. The inlet valve 19 is mounted in the low-pressure piston 8 and opens in flow direction to the low-pressure chamber 11 and closes pressure-tight in the opposite flow direction. The inlet valve 19 is so designed that it already opens in response to a low opening pressure.

[0022] The overflow channel 13 is disposed between the low-pressure chamber 11 and the high-pressure chamber 12. An overflow valve 20 is mounted in the overflow channel 13 and this valve opens in flow direction toward the high-pressure chamber 12 at a slight pressure difference and closes pressure-tight in the opposite direction. Furthermore, a spring-biased outlet valve 21 is disposed in the high-pressure chamber 12. The outlet valve 21 is accommodated in the smaller housing cover 3 and connects the high-pressure chamber 12 and therefore the second compressor stage to the outlet connection 18 leading to the consumer 48. The outlet valve 21 opens at a predetermined opening pressure in the flow direction toward the outlet connection 18 and closes pressure-tight in the opposite direction.

[0023] According to the invention, the low-pressure chamber 11 and the intake space 16 are connected to each other via a bypass line 22 which bypasses the inlet valve 19 of the low-pressure chamber 11. A pressure valve 23 is disposed in this bypass line 22 and is controlled by the pressure in the intake space 16. Correspondingly, the pressure valve 23 has a spring-biased valve element 24 and a pressure spring 25. The pressure spring 25 is so pretensioned that the valve element 24 continuously or intermittently clears an opening cross section in response to a predetermined pressure in the intake space 16 and thereby connects the low-pressure chamber 11 and the intake space 16 to each other and leads to a pressure equalization. To relieve pressure on the pressure valve 23, the valve element 24 is connected to the atmosphere via a leakage connection 26 at its spring-biased end. The compressor is designed with respect to power so that the first compressor stage with its low-pressure chamber 11 has a largest possible piston displacement which is adequate, for example, for filling a tire. In contrast, the second compressor stage has a piston displacement which, by itself, is adequate with a single-stage compression to supply the consumers of a closed air supply system with pressurized air. With this division of power of the two compressor stages, the power take-up capacity of the compressor lies above the maximum power limit of the electric drive 15.

[0024] For the purpose of compressing an airflow, the rotating movement of the electric drive unit 15 is converted by the crank gear 14 into an oscillating movement of the stepped piston 7. In this way, volume changes in the low-pressure chamber 11 and in the high-pressure chamber 12 result which cause a lower pressure to adjust in the low-pressure chamber 11 compared to the intake space 16 with an enlargement of the low-pressure chamber 11. This underpressure causes the inlet valve 19 to open and air to flow from the intake space 16 to the low-pressure chamber 11. Because of the subsequent reversal of the direction of movement of the stepped piston 7, there is a reduction of the volume in the low-pressure chamber 11 and therefore there is a pressure increase of the air enclosed therein. This pressure increase causes the inlet valve 19 of the low-pressure chamber 11 to close so that there is a first stage of the compression of the enclosed air. For a predetermined pressure, the overflow valve 20, which leads to the high-pressure chamber 12, opens and the compressed pressurized air is displaced from the low-pressure chamber 11 into the high-pressure chamber 12 with the continuing movement of the stepped piston 7. With the reversal of the direction of movement of the stepped piston 7, there is a pressure increase in the high-pressure chamber 12 at which the overflow valve 20 closes and a second stage of the compression of the compressed air is initiated. The outlet valve 21 opens at a predetermined pressure in the high-pressure chamber 12 and the compressed pressurized air reaches the consumer 48 via the outlet connection 18.

[0025] For the supply of an external consumer, for example, of a tire with air from the atmosphere or from a low-pressure pressurized-air store, the pressure valve 23 in the bypass line 22 remains closed because the force, which originates from the pressure in the intake space 16 and acts on one end of the valve element 24, is less than the force of the pretensioned pressure spring 25 which acts on the other end of the valve element 24. The compression of the inducted air then takes place in the manner described in two compression stages. The relatively high piston displacement in the first compressor stage makes a volume flow possible which is adequate for filling the tire, and the low pressure in both compressor stages keeps the power take-up of the compressor within the power range of the electric drive.

[0026] For supplying the consumers with pressurized air from a pressurized-air store, an intake pressure adjusts in the intake space 16, which corresponds to the pressure in the pressurized-air store and is higher than the atmospheric pressure. This intake pressure propagates likewise via the bypass line 22 and loads the valve element 24 of the pressure valve 23 against the force of the pressure spring 25. The pressure valve 23 is adjusted with a predetermined opening pressure. If the intake pressure in the intake space 16 remains below the opening pressure adjusted at the pressure valve 23, then the pressure valve 23 remains closed and the compressor operates with both compressor stages as described above. If the intake pressure reaches the opening pressure adjusted at the pressure valve 23, then the pressure valve 23 opens continuously or intermittently and connects both line connections at the pressure valve 23 and thereby the low-pressure chamber 11 and the intake space 16. For this reason, there is a continuous or intermittent pressure equalization between both chambers which is maintained over the entire movement of the stepped piston 7. In this way, the compression power in the low-pressure chamber 11 reduces continuously or intermittently until the first compression stage is no longer participating in the compression power of the compressor. For a fully opened pressure valve 23, the trapped air quantity is displaced without a further increase in pressure from the low-pressure chamber 11 into the high-pressure chamber 12, so that the compressor now operates exclusively with the second compressor stage. With the transition from the multi-stage to the single-stage compression, the power capacity of the compressor is throttled to a power value which does not exceed the power limit of the electric drive 15.

[0027] Referring to FIG. 1, the equalization of pressures in the compression stage can be initiated by the pressure in the intake space or the load pressure of a consumer 48 supplied by the compressor; or, electrically because of the loading of the on-board electrical system 46; or, a requirement of the control speed as detected in the ECU 44. Reference numeral 42 identifies an operator-controlled element connected to the ECU 44.

[0028] According to another feature of the invention and referring to FIG. 3, the control valve can be an electrically operated valve 32 which is connected via electric line 34 to the power control (ECU) 30 of the electric drive 15.

[0029] According to still another feature of the invention and referring to FIG. 4, the control valve can include a pneumatic control unit 36 which is connected to the outlet connection 18 via air line 38. Alternatively, the pneumatic control unit 36 can be connected as shown by the broken line 40.

[0030] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A method for limiting power of a compressor which compresses an air flow in at least two compression stages, the method comprising the step of causing the pressure in one of said compression stages to be adapted or equalized to the pressure in the other one of said compression stages when a predetermined power limit of said compressor is reached.
 2. The method of claim 1, wherein the equalization of pressures in said compression stages is initiated: pneumatically by the pressure in an intake space; by loading pressure of a consumer supplied by said compressor; or, electrically because of the loading of the on-board electrical system; or, a requirement of the control speed.
 3. The method of claim 1, wherein the pressure in said two compression stages is continuously equalized.
 4. The method of claim 1, wherein the piston displacement of the one compression stage is designed for the required volume flow of a consumer and the power of all non-pressure-compensated compression stages is matched to the power limit of the electric drive.
 5. A multi-stage compressor comprising: a compressor housing; a stepped piston disposed in said housing; said compressor housing and said stepped piston conjointly defining a first low-pressure chamber, a high-pressure chamber and a second low-pressure chamber; drive means for driving said stepped piston in oscillation; an inlet valve for connecting said first low-pressure chamber to said second low-pressure chamber; an outlet valve for connecting said high-pressure chamber to a consumer; an overflow valve for connecting said first low-pressure chamber and said high-pressure chamber to each other; a bypass line connecting said first low-pressure chamber to said second low-pressure chamber so as to bypass said inlet valve; and, a controllable control valve disposed in said bypass line and said control valve opening in a direction toward said second low-pressure chamber.
 6. The multi-stage compressor of claim 5, wherein said second low-pressure chamber is an intake space in said compressor housing; and, said first low-pressure chamber and said high-pressure chamber are part of respective first and second compression stages, respectively.
 7. The multi-stage compressor of claim 6, wherein said driver means is an electric drive; and, said control valve is set at an opening pressure which adjusts the power capacity of said compressor to the maximum power limit of said electric drive.
 8. The multi-stage compressor of claim 6, further comprising an outlet connection leading to said consumer; and, wherein said control valve includes a pneumatic control unit connected to either said intake space or said outlet connection.
 9. The multi-stage compressor of claim 6, wherein said drive means is an electric drive; and, said control valve includes an electric control unit which is connected to the power control of said electric drive. 